1. Field of the Invention
The present invention relates to a transmission diffraction grating element, a production method of diffraction grating element and a method of designing diffraction grating element.
2. Related Background of the Invention
Generally, a diffraction grating element is, in a transparent flat plate having a first plane and a second plane parallel to each other, formed with a diffraction grating on the first plane (see Kashiko Kodate, “Development of diffractive optics and future challenges”; Japan Women's University Journal, Faculty of Science, 10th Edition, pages 7 to 24 (2002), for example). In the diffraction grating element, for example, when light enters the first plane from a medium, which is in contact with the first plane, at a constant incident angle, the light is diffracted by the diffraction grating formed on the first plane, transmitted within the transparent flat plate, and then emitted to a medium, which is in contact with the second plane. The diffraction angle of the light, which is emitted from the second plane of the transparent flat plate, varies according to the wavelength.
Thus, the diffraction grating element may be used as an optical demultiplexer in which incident light is demultiplexed and then emitted. Also, when the light is guided in the direction opposite to the above case, the diffraction grating element may be used as an optical multiplexer in which incident light is multiplexed and then emitted. Further, by combining the diffraction grating element with another optical elements, a dispersion regulator, which regulates the group delay time of the light in accordance with the wavelength, may be constituted, for example. Accordingly, the diffraction grating element is one of the important optical elements in the wavelength division multiplexing (WDM) optical communication system, in which multiple wavelength signal light is multiplexed and then transmitted.
In the diffraction grating element, high diffraction efficiency is required. Some structural artifices for improving the diffraction efficiency have been proposed; and it is reported that about 95% diffraction efficiency has been achieved (see US Patent Application Publication No. 2002/0135876; Specification; and, Hendrick J. Gerritsen, et al., “Rectangular surface-relief transmission gratings with a very large first-order diffraction efficiency (−95%) for unpolarized light”, Applied Optics, Vol.37, No. 25, pp.5823-5829 (1998), for example).